Electricity is usually generated at one location and then transmitted to other locations where the electricity is used. From its very beginning the electric utility industry has been almost universally dependent on overhead transmission lines for transmitting bulk electric power. Recently, however, an alternative approach of using underground transmission is gaining increasing popularity but still has limited use due to its much higher cost, inferior thermal capacity and long repair time as compared to the overhead transmission. Based on current expenses and on an equivalent capacity basis, conventional underground transmission systems are still 10 to 30 times more expensive than overhead transmission systems.
It is projected that for yet another decade, the cost of overhead transmission will increase as a linear function of the voltage level. Beyond this period, however, the economics of overhead transmission will become more burdensome. In fact, there is a growing body of experimental evidence that practical overhead transmission substantially beyond 2 million volts will not be practical because of exorbitant expense.
Cost of underground transmission does not increase as rapidly with increasing voltage because certain large fixed costs, such as acquiring right-of-way and trenching, are essentially independent of voltage level. With the advent of extra high voltage (EHV) overhead transmission, i.e. 230 kv to 800 kv the cost ratio of EHV underground to EHV overhead for the first time in history will be significantly less than an order of magnitude, ranging anywhere from 3 to 6. As the voltage level increases into the ultra-high voltage (UHV) range, the cost ratio becomes even more favorable for underground transmission.
The utility industry is also under pressure from environmental groups to consider alternatives to overhead transmission. Environmentalists are frequently successful in gaining local support to oppose construction of new lines and as a result, some state regulations make it more difficult for utility companies to obtain approval of proposed construction of new overhead EHV and UHV lines. It appears that the situation will probably become even more tenuous in the future. For example, a 1500 kv overhead transmission line would require a right-of-way of approximately 400 feet. In many areas of the country, such a right-of-way would be very difficult if not impossible to obtain.
In view of the foregoing, the utilities have expended much time and effort in devising underground transmission systems. For example, in 1966 the Edison Electric Institute embarked on a research and development program in underground transmission which program is still continuing. Areas of research include compressed-gas-insulated (CGI), cryoresistive, and superconducting underground cables. Some CGI cable installations of short length have recently been made, but the cost of these cables is too high to make them economically feasible. Cryoresistive and superconducting cables are still in development stages. The disadvantages of such cables are that they all require cooling stations, which reduces their reliability, and the installation and repair of the cables will be very time consuming and costly. Many technical problems must still be overcome, and it is doubtful that superconducting cables will be commercially available before 1990.
The CGI, cryoresistive and superconducting cables utilize special atmospheres which are sealed within the conduits to obtain their large power-carrying capability. The initial installation costs of these cables is high and these systems must be exceedingly reliable in operation because of the large expenses incurred and the long down time needed to make repairs. For example, before any section of a conduit can be repaired, the conduit must be evacuated to remove the noxious special atmosphere so that repairman can work on the conduit. Therefore increasing emphasis is being placed on devising an underground transmission system for UHV and EHV voltage levels and which uses air as the only atmosphere within the conduit.
The advantages of using air rather than other gases as the insulative medium are manyfold. The conduits are readily accessible for workmen to enter since they are always free of noxious gases. Therefore inspection and maintenance are simplified. The air itself is free and it is not essential to employ special cooling stations or compressing stations to maintain the system in operation. None of these advantages inure to the CGI, cryoresistive and superconducting cable systems.
The cost of underground air transmission with the same power-carrying capability as overhead transmission can be reduced because its rated voltage can be substantially lower than that needed for the overhead line. For example, the power capability of a transmission line is proportional to the square of its voltage rating and is inversely proportional to its positive sequence surge impedance. For an underground system, this ranges from 50 to 150 ohms. This is substantially less than the 250 ohm level for overhead lines, and the power capability of the underground line can therefore be increased according to the described relation. For the same power capability, the obvious alternative is to reduce the rated voltage of the underground line by the square root of the ratio of the surge impedances. For the ohmic values given, this comes out to be .sqroot.100/250 = 0.63 so that a 37 percent reduction in voltage is possible for the underground line, with consequent attendant reduction in costs.
This basic idea which utilizes the normal atmosphere, underground air insulation system has been contemplated for use at 230 kv. In this type system, all three phases are contained in the same conduit and the arrangement is referred to as non-segrated phase bus. This is a well-known idea described, for example, in U.S. Pat. No. 3,349,168. When projected to 765 kv, however, this scheme is impractical because the conduit diameter would approach 30 feet.
By using an alternative well-known scheme called isolated phase bus, it is possible to reduce the conduit diameter to 15 feet. In this arrangement, three conduits, each carrying one phase bus, are installed side-by-side and examples of isolated phase bus systems of this type are found in U.S. Pat. Nos. 2,892,012 and 3,197,551.
However, it has been found that even the smaller conduits of 15 foot diameter are still too large to be economically feasible. Thus whether the non-segrated phase bus or the isolated phase bus systems are used, the conduit size is a very significant factor with respect to both cost and power capabilities. The conduit size varies directly with cost and power capability and there are currently no air systems available, neither non-segrated phase bus nor isolated phase bus systems, which can handle 765 kv levels.